Modified atmosphere package systems with gas-permeable plastic membranes and window for packaging of fresh fruits, vegetables and cut flowers in modified euro trays

ABSTRACT

This invention relates to novel package systems for refrigerated modified atmosphere packaging of fresh fruit, vegetables and cut flowers. More particularly, this invention relates to the design, construction, closure, sealing and use of gas-permeable paperboard package systems and shipping trays for prolonging the storage life of fresh fruits, vegetables and cut flowers under modified atmosphere in the headspaces of the closed package system. A gas permeable paperboard comprising: (a) a first layer of kraft paper; (b) a layer of polymer having a gas permeability which permits gas to be transmitted through the polymeric film at prescribed levels; (c) a second layer of kraft paper, said first and second layers of kraft paper sandwiching the polymer between them.

TECHNICAL FIELD

This invention relates to novel modified atmosphere package systems for packaging of fresh fruit, vegetables and cut flowers in Euro trays. More particularly, this invention relates to the design, construction, closure, sealing and use of modified atmosphere paperboard package systems with gas-permeable plastic membranes for loading into customized Euro trays for containing and prolonging the storage life and maintaining freshness of fresh fruits, vegetables and cut flowers using modified atmosphere in the headspaces of the closed package systems.

BACKGROUND

The quality attributes of fresh fruits, vegetables and cut flowers must be maintained as much as possible for as long as possible to ensure consumer acceptability. Quality deterioration of horticultural produce comes about through plant tissue enzyme reactions including respiration, ripening and senescence, through microbial growth and through water loss for the tissue. Methods of inhibiting the deteriorative enzyme reactions, and the growth of yeasts, molds and bacteria involve the reduction of the produce temperature to between 1° and 12° C., and the creation of low O₂/high CO₂ modified atmosphere (MA) around the produce. Water in fruits and vegetables can be lost readily under low relative humidity conditions with the consequence of skin wrinkling, wilting and reduction in crispness. The rate of water loss from produce can be restricted by storing the produce in closed package systems consisting of walls with low moisture permeability.

Corrugated paperboard boxes and cartons are used commercially for the storage and transport of flesh horticultural commodities. The advantages of the corrugated paperboard boxes and cartons are the relatively low cost per unit volume, low thermal energy conductivity, impact absorbing ability to prevent bruising of commodities and ease of disposal at the receiving end. One type of corrugated paperboard box is known as a Euro tray. It has a standard horizontal dimension of 40 cm width and 60 cm length.

Conventional corrugated paperboard has a very high gas and moisture permeability and as such is unsuitable for modified atmosphere packaging of horticultural commodities. The fresh produce industry incorporates the use of stackable corrugated fiberboard cartons or returnable plastic crates of various sizes and shapes to accommodate a wide array of fresh fruits and vegetables for transportation to market. In general, there are two broad categories of stackable fiberboard cartons used in the produce industry, namely open style cartons which incorporate apertures such as hand-holds and concavities for refrigerated air circulation and closed style cartons which do not include the apertures and concavities but incorporate selective gas permeable membranes to limit gas exchange between the sealed cartons and ambient atmosphere.

The main advantages of the open style cartons allows for direct field packing of the harvested produce in the cartons, followed by refrigeration and shipment to market. The simple packaging and cooling of the produce provides significant time, labor and cost savings. The main disadvantages of this type of packaging is that the free movement of oxygen around the produce maintains respiration rates and thus reduces the amount of time the produce can be stored and/or transported. To offset some of these deleterious effects, produce is harvested earlier in the growing season, while the produce is still green. This is before optimal nutritional values and desirable tastes have developed, thus reducing the quality of the produce delivered to market. Another disadvantage of the open style packaging is the minimal protection afforded to temperature fluctuations frequently encountered during transportation to market.

Closed style cartons, known also as modified atmosphere packaging (MAP), of fruits, vegetables and flowers is a process involving:

-   1. Pre-packaging conditions and treatment of produce; -   2. The packing of produce into a gas-permeable package system; -   3. The introduction of predetermined CO₂ and O₂ containing gas     mixture flush into the headspace of the package system or the     retention of existing air in the head-space of the package system;     and -   4. Closure and sealing of the MA package system.

During storage, the fruits, vegetables and flowers convert O₂ from the headspace to CO₂ through the respiration process with the result that the O₂ content in the headspace decreases while the CO₂ content increases. The objective in the design of an effective package system for MAP of produce is to regulate the influx of O₂ to an efflux of CO₂ from the package headspace to achieve and maintain a suitable equilibrium modified atmosphere in the headspace around the stored produce for optimum retention of the quality attributes and for the reduction of microbial growth while the fruits, vegetables and flowers are being shipped from source to destination, and storage at the destination.

Low O₂ levels and elevated CO₂ levels in the headspace around a horticultural commodity reduce the respiration and ripening rates, and the growth of spoilage organisms (spoilogens). Unsuitable modified atmospheres around produce in a package system can induce physiological damage, prevent wound healing, enhance senescence and cause off-flavor formation. O₂ levels lower than 1% bring about anaerobic respiration and off-flavor development, whereas CO₂ levels of about 10% or higher inhibit microbial growth but may cause tissue damage to CO₂-sensitive commodities.

Package systems for MAP must be designed and constructed with specific packaging materials to meet the following requirements:

-   1. Maintain definitive beneficial equilibrium levels of CO₂ and O₂     in the headspace; -   2. Obviate gas pressure build-up within a package system; -   3. Minimize moisture loss from produce; -   4. Prevent produce crushing and bruising; and -   5. Inhibit water migration from the package interior into the walls     of the package system for the purpose of retaining structural     strength of the walls.

The gas and moisture permeabilities of package components of MA package systems are critical parameters. The technology of plastic polymeric films has advanced to such an extent that a specific gas permeability requirement can be met with a single plastic film or a multifilm combination, with or without vent pinholes.

In 1960, Eaves (J. Hort. Sci. 37:110, 1960) reported the effectiveness of gas-permeable, flexible polymeric barrier film as a package system for extending the life of fresh commodities. Tomkins (J. Appl. Bacteriol. 25:290, 1962) used polymeric film-covered trays as package systems to determine their effectiveness in establishing equilibrium MA around apples. Prior art on the use of bags made from polymeric plastic, gas permeable film such as polyethylene and polyvinylchloride, for prolonging of shelf-life of stored fruits and vegetables, is exemplified by U.S. Pat. No. 3,450,542, Badran, U.S. Pat. No. 3,450,544, Badran et al., and U.S. Pat. No. 3,798,333, Cummin. A more complex package system has been described by Rumberger (U.S. Pat. No. 3,630,759) in that an inner plastic pouch containing the produce is enveloped by an outer pouch containing an atmosphere of less than 15% O₂ Both pouches are to be constructed from gas-permeable films.

U.S. Pat. No. 5,575,418, issued 19 Nov. 1996, Wu et al., relates to novel package systems for refrigerated modified atmosphere packaging of fresh fruit, vegetables and cut flowers. More particularly, the patent discloses the design, construction, closure, sealing and use of gas-permeable corrugated paperboard package systems for prolonging the storage life of fresh fruits, vegetables and cut flowers under modified atmosphere in the headspaces of the closed package system. The corrugated gas permeable paperboard comprises: (a) a first layer of kraft paper; (b) a layer of polymer having a gas permeability which permits gas to be transmitted through the polymeric film at prescribed levels; (c) a second layer of kraft paper, said first and second layers of kraft paper sandwiching the polymer between them; (d) a corrugated fluting; and (e) a third layer of kraft paper affixed to the corrugated fluting.

U.S. Pat. No. 5,609,293, issued 11 Mar. 1997, Wu et al., relates to the design, construction and use of lined or coated corrugated paperboard package systems (e.g. boxes, cartons) for prolonging the storage life of fresh fruits and vegetables under modified atmospheres (MA) in the headspaces of the closed package systems. The plastic-paperboard construction comprises a first layer of polymeric film, a second layer of kraft paper adjacent the first layer, a kraft paper corrugated flute adjacent the second layer and a fourth layer of kraft paper adjacent the flute.

U.S. Pat. No. 6,050,412, issued 18 Apr. 2000, Clough, et al., relates to a novel method and apparatus for packaging and shipping horticultural products including cut flowers. In particular, the patent discloses a novel method of and packaging for packaging cut flowers in a modified atmosphere package to prolong shelf life, shipping the packaged flowers to the destination, and then at the destination, opening the package and rehydrating the cut flowers in the package by saturating the stems of the flowers with water.

None of the cited references discloses stackable closed packages which incorporate modified atmosphere technology, allow cooling ventilation of stacked and/or palletized packages or facilitate visual inspection of the package contents without having to open the sealed package.

There is a need for a closed stackable package system which incorporates modified atmosphere technology, allows harvesting and packaging of produce in either the field or in a centralized facility, allows cooling ventilation of stacked and/or palletized packages directly in the modified atmosphere packaging, allows visual inspection of the packaged produce at any point following packaging and provides the ability to re-establish refrigeration of the produce during transit.

The foregoing examples of the related art and limitations related thereto are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.

SUMMARY OF THE INVENTION

The following embodiments and aspects thereof are described and illustrated in conjunction with systems, tools and methods which are meant to be exemplary and illustrative, not limiting in scope. In various embodiments, one or more of the above-described problems have been reduced or eliminated, while other embodiments are directed to other improvements.

The invention is directed to a closed gas permeable paperboard package comprising a box and a lid, each constructed of: (a) a first layer of kraft paper; (b) an intermediate layer of gas permeable polymer; and (c) a second layer of kraft paper, said first and second layers of kraft paper sandwiching the polymer between them, said paperboard box and lid each having a gas permeability which permits gas to be transmitted through the paperboard box and lid at prescribed levels.

The gas permeability of the polymer or paperboard can be between about 50 and 100,000 cc³/m² 24 hr. 1 atm. The first layer of kraft paper, the layer of polymer and the second layer of kraft paper can be formed together at the same time. Alternatively, the layer of polymer can be extruded on one side of the first layer of kraft paper and then affixed to the second layer of kraft paper and then laminated between the first layer of kraft paper and the second layer of kraft paper. The layer of polymer or coating can be low density polyethylene or high density polyethylene, or a copolymer of low density polyethylene and ethylenebutyl-acetate, or other polymers.

The gas permeability of the paperboard can be regulated by dictating the amount and composition of the polymer, or by regulating the rate of extrusion of the polymer in forming the layer of polymer.

The invention is also directed to a gas-permeable modified atmosphere paperboard combination comprising: (a) a first layer of kraft paper; (b) a layer of controlled gas permeable polymer with no natural pinholes therein; and (c) a second layer of kraft paper, said first and second layers of kraft paper sandwiching the layer of polymer between them.

The gas permeability of the polymer can be between about 50 and 100,000 cc³/m² 24 hr. 1 atm. The first layer of kraft paper, the layer of polymer and the second layer of kraft paper can be affixed together in one step. Alternatively, the layer of polymer can be extruded on one side of the first layer of kraft paper and the second layer of kraft paper can then affixed to the layer of polymer on the side opposite the first layer of kraft paper. As a further alternative, the layer of polymer can be preformed and can then be laminated between the first layer of kraft paper and the second layer of kraft paper.

The layer of polymer can be low density polyethylene or high density polyethylene, or a copolymer of low density polyethylene and ethylenebutylacetate. The polymer can be selected from the group consisting of ethylene vinylacetate (EVA), ethylbutyl acetate (EBA), a crosslinked ionomer resin, cast polyester (PET), a polyamide and polycarbonate (PC).

The overall permeability of the paperboard combination can be regulated by regulating the thickness of the polymer and the processing conditions in forming the paperboard combination so that no natural pinholes are formed. The overall permeability of the paperboard combination can be regulated in part by regulating the composition of the polymer layer and the type of kraft paper.

A portion of the first layer of kraft paper and a corresponding opposite portion of the second layer of kraft paper can be absent so that the intermediate polymer layer forms a window. The polymer layer at the location of the window can include an anti-fog agent. The anti-fog agent can be silicone, non-ionic surfactant or an oil-based material.

The invention is also directed to a paperboard modified atmosphere package container suitable for packaging fruits and vegetables under refrigerated modified atmosphere conditions comprising: (a) a container constructed of an erected paperboard blank having flaps, side panels, end panels, base panel and lid panel with flaps, side panels and end panels formed therein and fold and joint lines impressed therein, said paperboard blank being constructed of: (i) a first layer of kraft paper; (ii) a layer of polymer having a gas permeability which permits gas to be transmitted in either direction through the polymer at prescribed levels; and (iii) a second layer of kraft paper, said first and second layers of kraft paper sandwiching the polymer between them; and (b) glue applied to intersecting joints, flaps, side panels, end panels, and lid panels to provide a hermetic seal to the erected container, said container having a required overall permeability which permits gas to be transmitted into or out of the container.

The flaps, side panels, end panels and base panel of the container can be formed from one piece of paperboard and the lid panel with flaps, side panels and end panels can be formed from a second piece of paperboard.

The glue can be hot melt glue which can be applied to cover exposed and intersecting edges of the joints, flaps, side panels, end panels and lid panels. The glue can be a foamed glue which can be applied to cover exposed and intersecting edges of the joints, flaps, side panels, end panels and lid panels. The glue can be cold set water resistant glue which can be applied to cover exposed and intersecting edges of the joints, flaps, side panels, end panels and lid panels.

At least some edges of the joints, flaps and panels can be sealed with tape. At least some of the joints, flaps and panels can be sealed with glue and tape. A first side of the first layer of kraft paper, opposite the side adjacent the polymer layer, can be coated with a polymeric coating. Edges of flaps intersecting with sides of the container can be sealed with glue, and the flaps can be bent around the corners of the glued edges, and can be glued at a second location.

A portion of the first layer of kraft paper and an adjacent portion of the second layer of kraft paper can be absent to thereby form a window in the container. The lid panel can have a window. The window can incorporate or have on its surface an anti-fog agent. The anti-fog agent can be silicone, non-ionic surfactant or an oil-based material.

The layer of polymer can be at least partially permeable to oxygen and carbon dioxide. Exposed edges of flaps and sides can be folded, and the exposed edges of the paperboard can be sealed with glue.

The layer of polymer can be selected so that the container can have a gas permeability which is within a predetermined gas transmission range which is selected in accordance with the respiration rate and characteristics of produce to be packaged in the container. The layer of polymer can be flexible and can have selected gas and moisture permeabilities. The gas permeability of the polymer can be between about 50 and 100,000 cc³/m² 24 hr. 1 atm. The container can be filled with produce, a modified atmosphere can be injected into the container, and the container can be held at a temperature between about 1° C. to 12° C.

The invention also pertains to a paperboard modified atmosphere package container combination suitable for packaging fruits, vegetables and cut flowers under refrigerated modified atmosphere conditions comprising: (a) a container constructed of erected paperboard lid and body blanks having joints, flaps, side panels, end panels, base panels and lid panels formed therein and fold and joint lines impressed therein, said paperboard blanks being constructed of: (i) a first layer of kraft paper; (ii) a layer of polymer having a gas permeability which permits gas to be transmitted in either direction through the polymer at prescribed levels; and (iii) a second layer of kraft paper; glue applied to intersecting joints, flaps, side panels, end panels, base panels and lid panels to provide a hermetic seal to the erected container; and (b) a Euro tray holding the container.

A plurality of erected containers containing fruit or vegetables can be positioned in the Euro tray. The Euro tray can have recessed ends and sides to facilitate circulation of cool air when the Euro trays are stacked upon one another.

In addition to the exemplary aspects and embodiments described above, further aspects and embodiments will become apparent by reference to the drawings and by study of the following detailed descriptions.

BRIEF DESCRIPTION OF DRAWINGS

Exemplary embodiments are illustrated in referenced figures of the drawings. It is intended that the embodiments and figures disclosed herein are to be considered illustrative rather than restrictive.

FIG. 1 illustrates an isometric view of an erected and assembled three-layer linerboard to construct a two-piece MAP box base and lid.

FIG. 2 illustrates an isometric view of an erected three-layer linerboard to construct a one-piece MAP box base and lid.

FIG. 3 illustrates an isometric view of a three-layer linerboard erected in the shape of a bag.

FIG. 4 illustrates a plan view of a three-layer linerboard blank of a square bottom for a two-piece MAP paperboard package.

FIG. 5 illustrates a plan view of a three-layer linerboard blank of a square lid for a two-piece MAP paperboard package.

FIG. 6 illustrates a plan view of a three-layer linerboard blank of a rectangular bottom for a two-piece MAP paperboard package.

FIG. 7 illustrates a plan view of a three-layer linerboard blank of a rectangular lid for a two-piece MAP paperboard package.

FIG. 8 illustrates a plan view of a three-layer linerboard blank of a square bottom and lid for a one-piece MAP package.

FIG. 9 illustrates a plan view of a three-layer linerboard blank of a rectangular bottom and lid for a one-piece MAP package.

FIG. 10 illustrates a side section view of a portion of a three-layer linerboard, with a window area that is the polymer layer without kraft paper layers on either side of the polymer layer.

FIG. 11 illustrates a plan view of a blank for a modified Eurobox.

FIG. 12 illustrates an isometric view of a modified Eurobox with recessed sides and ends.

FIG. 13 illustrates an isometric view of a modified Eurobox holding a series of MAP packages according to the invention.

FIG. 14 illustrates a plan view of a three-layer linerboard blank for a MA bag, as shown in FIG. 3.

DESCRIPTION

Throughout the following description specific details are set forth in order to provide a more thorough understanding to persons skilled in the art. However, well known elements may not have been shown or described in detail to avoid unnecessarily obscuring the disclosure. Accordingly, the description and drawings are to be regarded in an illustrative, rather than a restrictive, sense.

This invention pertains to the design, construction, closure, sealing and use of novel consumer friendly paperboard package systems (e.g. boxes, cartons) formulated to include plastic coating or film of selected gas permeability as part of the wall structure for prolonging the storage life of fresh fruits, vegetables and flowers under modified atmosphere (MA) in the headspaces of the closed package systems, and retail purchase by consumers.

Due to the long distance and the time required for continental and overseas shipment of MAP preserved fruits, vegetables and flowers, we have invented a unique three-layer linerboard which can be cut, folded and formed into a consumer sized box bottom and lid which in turn can fit into a modified Euro tray (60×40 cm) which can be used for the shipment of tomatoes, grapes, peaches and nectarines. Typically, the box bottom and lid are of a size that can contain four to six peaches, which is a convenient quantity for a consumer to purchase. Thus the MA box bottom and lid, being sealed, does not have to be opened until it reaches the consumer's home, thereby preserving the produce contents. The modified Euro tray, according to the invention, is an open tray with air circulating recessed sides and ends and has strong corner supports which enables the modified Euro trays to be stacked on a pallet.

A common request from customers of packaged fruits, vegetables and flowers is a capability to view the packaged product without opening the package. A window in the package is desirable. However, since the packaged product is refrigerated and since moisture is present in the interior of the package from fruit or vegetables, the window, if there is one, usually fogs up from condensation. A problem with conventional corrugated modified atmosphere packaged fruit, vegetables or flowers is that the corrugated paperboard construction provides insulation, which retains heat. If the fruit or vegetable product in the package is warm, it requires time to cool down to acceptable modified atmosphere refrigeration levels. A window in the package is useful because it transmits respiration heat from the packaged produce, whereas linerboard inhibits heat transmission. A window also enables the produce to be cooled quickly to refrigeration temperatures if the produce is initially at room temperature.

The weight of the fruits or vegetables to be packaged can be in a range from small such as 100 g to a large package such as 5 kg (equivalent of a wholesale pack). The MA packages are adapted to include fruits, vegetables, fresh-cut flowers, meat and fish. Currently, paper companies manufacture coated paper products or laminated papers. However, the purpose or usage of such papers is mainly for water repellent or physical strength.

The three-layer linerboard blanks for the MA package, according to the invention, have sufficient physical strength that they can be constructed into boxes or bags with a clear window. The linerboard blanks can be cut with appropriate fold lines to form either a two-piece container (box base and lid) or a one-piece container with the lid flap folding over the open top of the box base. Square, rectangular and other shapes are possible. The openings between the lid and the bottom are taped or glued to seal the package completely in order to provide an MA atmosphere. The linerboard blanks can be formed so that the lid area has a window, which is created by not having a first or second layer of kraft paper obscuring the polymer film in the window area. The polymer film layer for the window should be reasonably clear and contain or be treated with an anti-fog agent. The glue used to seal the MA package must be waterproof. With the box construction, the linerboard must be of a specified weight to provide the appropriate strength. The linerboard weight can be about 100 to 140 g/m². However, a larger range is included in the scope of the invention for wholesale and other applications.

For a larger wholesale bag application, the paper weight can be reduced but the polymer must remain approximately the same for gas transmission purposes. The thinner linerboard for bags is more pliable and can be formed on a packaging machine into paper bags without sacrificing gas permeability. The weight of the linerboard can be 140 g/m² but broader ranges are included in the scope of the invention.

Specific embodiments of the invention will now be discussed in association with the drawings. FIG. 1 illustrates an isometric view of an erected and assembled three-layer linerboard to construct a two-piece MAP box and lid. As seen in FIG. 1, the two-piece MA box bottom and lid 2 is constructed of a lid 4 which fits snugly on a box bottom 6. The adjacent two layers of kraft paper on each side of the intermediate polymer film layer for the lid 4 are absent in the central area of the lid to create a window 8.

FIG. 2 illustrates an isometric view of an erected three-layer linerboard to construct a one-piece MAP box bottom and lid. As seen in FIG. 2, the one-piece MA box bottom and lid is constructed with a lid 12 and a window 14, the lid 12 folding snugly over a box base 10. Side flaps 22 extend downwardly from the lid 12 on three sides and fit snugly on the sides of the box base 10. The hinge for the lid 12 is at crease 11.

FIG. 3 illustrates an isometric view of a three-layer linerboard erected in the shape of a bag. As seen in FIG. 3, the bag has an upright body 16 with a peaked top 20.

FIG. 4 illustrates a plan view of a three-layer linerboard blank of a box bottom for a two-piece MAP paperboard package. As seen in FIG. 4, the box base 6 is formed from the three-layer linerboard in a square pattern, with fold lines 22 which define the box bottom 6 and side flaps 24. When erected along the fold lines 22, a square shaped box bottom 6 is formed, as shown also in FIG. 1.

FIG. 5 illustrates a plan view of a three-layer linerboard blank of a lid for a two-piece MAP paperboard package. As seen in FIG. 5, the lid 4 is formed from the three-layer linerboard blank and has thereon a central square window 8, fold lines 26 which define the central area of the lid 4 and surrounding sides 28.

FIG. 6 illustrates a plan view of a three-layer linerboard blank of a rectangular bottom for a two-piece MAP paperboard package. FIG. 7 illustrates a plan view of a three-layer linerboard blank of a rectangular lid for a two-piece MAP paperboard package.

FIG. 8 illustrates a plan view of a three-layer linerboard blank of a box bottom and lid for a one-piece MAP package. As seen in FIG. 8, the one-piece MA box base and lid is formed from a three-layer linerboard blank to define a box bottom 10, an adjoining lid 12, with window 14. A series of fold lines 30 define box bottom sides 24 and lid sides 22.

FIG. 9 illustrates a plan view of a three-layer linerboard blank of a rectangular bottom and lid for a one-piece MAP package.

FIG. 10 illustrates a side section view of a portion of a three-layer linerboard with a window area that is formed by having ki aft paper layers absent on each side of the polymer layer. As seen in FIG. 10, which illustrates a three-layer linerboard construction, the linerboard is constructed of a first layer of kraft paper 34, an intermediate layer 36 of polymer film, preferably transparent, and a second layer of kraft paper 38. In the window area, the polymer film 36 is exposed, by removing the first and second layers of the kraft paper 34, 38 on the opposite sides of the window 8.

FIG. 11 illustrates a plan view of a blank for a modified Eurobox. As seen in FIG. 11, the Eurobox 40 is erected from a cardboard blank which has a series of fold lines 42. The box 40 has recessed sides 44 and ends 46.

FIG. 12 illustrates an isometric view of a modified Eurobox with recessed sides and ends. As seen in FIG. 12, the modified Euro tray 40 is sized to standardized shipping container specifications, namely 40 cm×60 cm, and has recessed sides 44 and recessed ends 46. In the embodiment shown in FIG. 12, the tray 40 also has reinforced corners 48, which enable the trays to be stacked. The recessed sides 44 and ends 46 enable refrigerated air to be circulated through both the sides and ends of stacked Euro trays 40.

FIG. 13 illustrates an isometric view of a modified Eurobox holding a series of MAP packages. As seen in FIG. 13, the modified Euro tray 40, with reinforced corners 48 and recessed sides 44 and ends 46, is filled with rows of MA box bottoms and lids 2. The MA packages 2 are loaded with fresh fruit, vegetables or flowers. It will be recognized that while twelve MA containers are shown in FIG. 12, the sizes of the containers can be varied so that larger MA containers can fill the modified Euro tray 40, such as six or eight MA packages to a Euro tray.

FIG. 14 illustrates a plan view of a three-layer linerboard blank for a MA bag, as shown in FIG. 3. As seen in FIG. 14, the linerboard blank for the bag has a front side 48 with window 18, a rear side 50, a pair of bottom panels 52 and a pair of top panels 54 which when folded together form the peaked top 20 (see FIG. 3). The dotted lines 56 indicate fold lines. The bag is erected by folding the blank along the fold lines. The overlapping panels are glued together to form the sealed upright MA bag.

Storage life of fresh fruits and vegetables is dependent on storage temperature, gas composition around the produce and degree of physical abuse leading to bruises, abrasions and cuts. Storage and transportation of fruits and vegetables is facilitated by the packing of the produce in suitable package systems which provide features such as prolonging storage life, reducing physical abuse and lowering the rate of water loss of produce.

Corrugated paperboard boxes and cartons are used commercially for the storage and transportation of fresh fruits and vegetables for the following reasons:

-   1. Relatively low cost per unit volume; -   2. Low thermal conductivity; -   3. Impact absorbing ability to prevent produce bruising; -   4. Ease of disposal at the receiving end; and -   5. Moderate stacking strength.

Since corrugated paperboard has very high O₂ and CO₂ permeabilities, this material by itself would be unsuitable for the construction of MA package systems. By incorporating a plastic, gas-permeable membrane into the corrugated paperboard structure, suitable MA package systems with specific gas and moisture permeabilities can be constructed.

The tri-layer linerboard comprising polymer film 30, and paper layers 28 and 32 (see FIG. 6) is formed by securing the film 30 between two layers of paper 28 and 32 by gluing or heat treatment.

The tri-layer liner with the polymeric plastic film membrane 30 as the middle layer will prevent water movement from the inside cavity, filled with a fruit or vegetable, with the benefit of the retention of the original wall strength. A gas-permeable, flexible polymeric plastic film with specific gas and moisture permeabilities is suitable for placement between two sheets 28 and 32 of kraft paper to form a tri-layer complex with specific O₂ and CO₂ and moisture permeabilities. The tri-layer liner may be manufactured by any of the following production methods:

Extrusion Lamination

This is a process whereby a molten polymer is extruded through a slit die and applied as a laminant to combine the two kraft paper substrates. By employing extrusion lamination, it is possible to produce very thin calliper films thereby producing a material with high permeability characteristics. Such thin polymer films would not be practical if produced as separate film liners or bag-in-box. By laminating to kraft paper, physical support is provided to protect the thin polymer film.

Extrusion Coating

This is a process whereby a molten polymer film is extruded through a slit die onto one kraft paper substrate and in a second operation, adhesive lamination is employed to combine the second kraft liner.

Adhesive Lamination

This is a process whereby a pre-made polymer film, produced by slit die extrusion or annular die film blowing, is adhesive laminated to the two kraft substrates, either simultaneously or in sequence.

The gas permeable polymeric layer can be homopolymers or copolymers produced as a monolayer or coextruded layers with specific formulation and caliper selected to produce the required oxygen (O₂) and carbon dioxide (CO₂) permeabilities. Polymers would likely be selected from the polyolefin family, typically Low Density Polyethylene (LDPE), linear low density polyethylene (LLDPE), medium and high density polyethylene (MDPE and HDPE), polypropylene (PP). Additional polymers such as ethylenevinylacetate (EVA), ethyl butyl acetate (EBA), ionomer resins (cross-linked), cast polyester (PET), nylon (polyamide) and polycarbonate (PC) may also be considered.

Coextrusions combining low density polyethylene with ethylenevinylacetate or ethylbutylacetate have been found to be particularly effective in lowering gas barrier to produce a highly permeable film. Percentages of ethylenevinylacetate or ethylbutylacetate are at the range of 5% to 30%.

A further unique embodiment of this invention is the ability of the box to maintain its internal equilibrium volume under varying gas compositions in the headspace. The gas permeability of the box prevents a vacuum condition developing which can occur in conventional MAP systems if the package produce starts to absorb carbon dioxide. If such conditions were to develop in the permeable box, the controlled influx of gases through the gas permeable film would not allow a vacuum to develop.

For specific product applications, the rate of gas exchange within the box may be achieved by a combination of polymer barrier and controlled film porosity. Porosity may also be achieved by piercing holes through the polymer containing inner liner either at the corrugating stage, die cutting operation, box forming stage, or in the completed box. Hole size, either single or multiple, may vary depending on the required gas exchange rate but typical diameter would be in the range of 0.25 to 2.00 mm. Hole positions on the box will vary depending on the optimum location for each product and the gas flow dynamics within the box.

It has been established that oxygen (O₂) and carbon dioxide (CO₂) gas exchange rates through the tri-layer paperboard of the invention fall within the range 50-100,000 cc³/m² 24 hr. 1 atm.

It has also been found that the following additional factors must be critically controlled if consistent polymer characteristics are to be achieved:

-   Process conditions: Extrusion rate, melt temperature, melt pressure,     nip pressure, nip position, chill roll temperature, corrugating     process conditions, board die cutting conditions. -   Polymer characteristics: Melt flow index, additives (processing     aids). -   Paper characteristics: Fibre length, virgin or recycled pulp, smooth     or rough side, with or without calender process.

In this invention, the plastic film membrane 30 is sandwiched between two sheets 28 and 32 of kraft paper to form a tri-layer complex as the inner liner of a paperboard MA package system. The film membrane 30 may be a gas-permeable plastic film or a plastic coating applied to one of the sheets of kraft paper, and then sealed between the two sheets. The membrane is bonded to both of the kraft paper sheets when the plastic is in semi-molten state and the two paper sheets are pressed together.

When 25 g/m² low density polyethylene was used, extrusion laminated on 40 g/m² and 125 g/m² MG kraft, the O₂ and CO₂ permeability were 1300 and 2200 cc/m² 24 hr. 1 atm. respectively. When a 35 g/m² coating of 17% EBA and LDPE was extrusion laminated, the O₂ and CO₂ permeability were 2300 and 4700 cc/m² 24 hr. 1 atm. respectively.

Studies have been carried out on the MAP of fruits and vegetables with sealed polymeric, plastic film bags in a corrugated paperboard box (Prince, 1989). However, several disadvantages of using a bag-in-box are evident:

-   1. Loss of headspace around produce with the shrinkage of the bag     under negative pressure created by respiratory CO₂ dissolution in     produce tissue; -   2. Extra handling of two packages, namely, the bag and the box; -   3. The thickness of the bag film must be at least 6 mil to ensure     bag durability during the handling, and thus high package cost and     low gas permeability of the bag is unavoidable. -   4. Thin plastic film serves as a cold surface for condensation of     water emitted from the produce, and can result in weight loss of     produce.

The single-piece and three-piece types of MA package systems are to be constructed in such a manner that upon gluing, folding and pressing at glue points, the following requirements are met:

-   1. The inside surface of the package systems are smooth with no     paper board projections. -   2. The package systems must be resistant to stacking-pressure     collapse. -   3. The MA package systems are airtight, yet the walls have specific     O₂ and CO₂ permeabilities.

The single-piece type MA package system is intended to be used on a continuous-flow or a batch-type operation consisting of:

-   1. A gluing, folding and pressing operation to create an open box; -   2. The filling of the open box with produce; -   3. Gluing, closing and sealing of box lid and flaps; -   4. Gasification of produce headspace.

The single-piece type MA package system may have gas inlet and gas outlet apertures in the two end panels for gasification of the produce headspace in a completely closed MA package system (including glued, sealed flaps).

A further benefit of injecting gas into an hermetically sealed box is that it is possible to include a trace gas, typically helium or sulphur hexafluoride as a leak detection method. Provided the box is relying on gas permeability and not porosity, it is possible to sense gas escape through cracks, unwanted pinholes or faulty glue seals.

Upon the insertion of a gas nozzle into the inlet aperture and upon the flow of the pressurized gas mixture through the headspace of the package system, plugs with vent pinholes or styrofoam plugs would be used for produce with high respiration rates and gas-impermeable plastic plugs may be used for low respiration rate produce.

Also, the two-piece type MA package system is intended to be used on a continuous-flow or a batch-type operation consisting of:

-   1. A gluing, folding and pressing operation to create an open box; -   2. The filling of the open box with produce; -   3. Gluing; -   4. Gasification of produce headspace.

The three-piece type package system may have gas inlet and gas outlet apertures in the two end panels for gasification of the produce headspace in a completely closed MA package system. Upon the insertion of a gas nozzle into the inlet aperture and upon the flow of a pressurized gas mixture through the headspace of the package system, plugs with vent pinholes or styrofoam plugs are to be inserted. Styrofoam plugs would be used for produce with high respiration rate produce, and gas-impermeable plastic plugs or gas-impermeable tape may be used for low respiration rate produce.

The selection of either the single-piece type or the three-piece type will depend upon:

-   1. Unit cost with respect to the amount of waste paperboard in the     die-cutting operation and the number of die-cutting operations per     unit package system; -   2. The complexity of the closing operation; -   3. Adaptability to the gasification operation; -   4. Strength of the closed package system (resistance to stack     compression); and -   5. Ease of handling, stacking and palletizing.

EXAMPLE 1 Determination of Permeability of Barrier Materials Manufactured with Different Kinds and Amounts of Polymers

In the applicant's research, barrier materials were manufactured according to the following procedures:

1. Material Descriptions

The tri-layer samples were constructed using 40 g/m² MG kraft+Polymer+125 g/m² MG kraft liner. Two kinds of kraft paper were used: 125 g/m² and 40 g/m². The smoothness (roughness) measurements of both sides were as follows:

Kraft Paper Inside Outside 125 g/m² 600 (bendtsen) 1,000 (bendtsen)  40 g/m² 125 (bendtsen)   420 (bendtsen

Three polymer materials, low density polyethylene (LDPE), and high density polyethylene (HDPE), a copolymer of LDPE and ethylene butyl acetate (EBA) at 10 g, 15 g, 25 g, 35 g and 45 g per square meter were used.

2. Process Conditions

At fixed process conditions, extrusion polymer melting temperature was 315° C. and the air gap (or nip height) was 200 mm.

Using the smooth or rough side of the kraft paper, the relative permeabilities and pinhole numbers on the flat sheet, or the folding lines are tabulated below in Table 1.

TABLE 1 Relative Permeability Obtained from Barrier Materials Manufactured by Various Polymers and Conditions Pinhole Numbers Coating Folding Polymer Relative Flat Line Folding Coating Weight Permeability Liner/ Width/ Line Material Side (g/m²) O₂ CO₂ m² m Length/m LDPE Smooth 10 3.58 3.46 225 79 33 15 1.70 1.68 38 2 2 25 1.04 1.08 13 1 0 35 1.03 1.06 0 0 0 45 1 1 0 0 0 LDPE Rough 10 4.80 4.64 TNTC 84 77 15 1.83 1.76 1,312 10 21 25 0.90 0.89 0 1 0 35 0.99 1.02 0 0 0 45 0.94 0.95 0 0 0 LDPE Smooth 10 17.26 16.52 2,800 125 111 & EBA 15 6.21 6.06 600 57 32 25 2.76 2.70 0 0 0 35 1.99 1.95 0 0 0 45 1.89 1.87 0 0 0 HDPE Smooth 10 3.23 3.11 100 70 29 15 1.17 1.13 30 13 3 25 0.74 0.77 0 0 0 35 0.63 0.66 0 0 0 45 0.60 0.61 0 0 0

A number of conclusions can be drawn from the results in Table 1:

-   1. The higher gas permeability of 10 g/m² and 15 g/m² materials was     due to pinholes in the polymer layer formed during the manufacturing     process. -   2. The relationship of thickness of the polymer layer and the gas     permeability of that material do not necessarily follow the general     principle of polymer film, ie. that the gas permeability of the film     is inversely proportional to the thickness. At the coating polymer     weight between 20 g-45 g/m², the gas permeability is mainly governed     by the polymer type and the process conditions, but is less affected     by the amount of polymer used. -   3. When the smooth side of MG paper was used for polymer coating,     the gas permeability was more consistent, and less pinholes were     formed.

EXAMPLE 2 Gas Permeability of Barrier Materials Manufactured at Different Processing Conditions

When extrusion lamination process conditions were changed, but the kinds and amounts of polymer and kraft liner stayed the same (as in Example 1) it was found that the gas permeability of the barrier materials manufactured under various process conditions were tabulated in Table 2.

TABLE 2 Relative Gas Permeability of Barrier Material Manufactured Under Various Process Conditions (Using 25 g/m² LDPE at 315° C. as Standard) Extrusion Polymer Material Temperature Nip Height Relative Permeability and Weight ° C. (mm) O₂ CO₂ LPDE 285 280 1.15 1.12 25 g/m² 285 180 1.25 1.24 315 200 1 1 330 280 0.70 0.71 330 180 0.81 0.80

The above results lead to the following conclusions:

-   1. Melting (extrusion) temperature has a greater effect on the     barrier property than the air gap (nip height). -   2. Consistent gas permeability can only be obtained under strictly     controlled processing conditions.

EXAMPLE 3

Although by using different polymers and process conditions, barrier materials of various permeabilities can be achieved, for very high respiring produce, high permeability materials need to be developed. In order to precisely control the permeability to match the need of certain specific produce, and box configurations (volume/surface area ratio), the permeability of a barrier material can be achieved by making controlled pinholes.

For specific product applications, the rate of gas exchange within the box may be achieved by a combination of polymer barrier and Controlled film porosity. Porosity may be achieved by piercing holes through the polymer containing inner liner either at the corrugating stage, die cutting operation, or box forming stage. Hole size, either single or multiple, may vary depending on the required gas exchange rate but typically would be in the range of 0.25 to 2.00 mm diameter. Hole positions on the box will vary depending on the optimum location for each product and the gas flow dynamics within the box.

The pinhole size and pinhole numbers will affect the resulting final gas permeability. Table 3 below gives examples of using different sizes of pinholes to achieve same open areas and relative permeabilities.

TABLE 3 Relative Gas Permeabilities Obtained by Same Porous Area Created by Different Pinhole Sizes and Pinhole Numbers Relative Permeability (cc/m², 24 hr. atm.) Pinhole Size (mm) Pinhole Number O₂ CO₂ 0.3 25 1 1 0.6 6 1.35 1.40 1.1 2 1.90 1.77 1.5 1 0.60 0.59 0.3 50 1.47 1.47 0.6 12 2.00 2.08 1.1 4 2.68 2.55 1.5 2 2.60 2.73

EXAMPLE 4

A box (dimension 56×39×19 cm) made of paperboard and tri-layer barrier liner G was filled with 20-21 lb. fresh broccoli crowns. The permeability of this MAP package was found to be very close to the broccoli produce's need but not exactly right. Therefore the controlled pinhole method was used to improve the gas permeability packaging condition. The gas composition in the headspace and the quality of the broccoli product are presented in the following table.

TABLE 4 Storage Box with Liner G + 8 mm Time Box with Liner G Pinholes (2° C.) O₂ CO₂ Product Quality O₂ CO₂ Product Quality 1 11.40 6.34 very good 15.46 5.76 very good 2 4.90 11.53 very good 13.36 6.73 very good 3 1.00 15.06 very slightly 11.88 9.27 very good off odor 4 1.22 17.39 moderate off 11.21 10.00 very good odor 7 1.22 17.75 moderate 11.66 9.82 very good strong off odor 8 0.47 21.71 strong off odor 11.58 9.87 very good

The following table provides dimensions for the blanks of typical two-piece and one-piece MA packages that come within the scope of the invention:

TABLE 5 Size per Euro Tray Length (cm) Width (cm) Height (cm) 6 boxes (one piece) 62.5 38.9 10.2 8 boxes (one piece) 53.7 38.7 10.2 6 boxes (two piece) Bottom 38.9 38.9 10.2 Lid 28.7 28.7 5.0 8 boxes (two piece) Bottom 38.7 34.5 10.2 Lid 28.5 24.3 5.0

While a number of exemplary aspects and embodiments have been discussed above, those of skill in the art will recognize certain modifications, permutations, additions and sub-combinations thereof. It is therefore intended that the following appended claims and claims hereafter introduced are interpreted to include all such modifications, permutations, additions and sub-combinations as are within their true spirit and scope. 

1. A gas-permeable modified atmosphere paperboard combination comprising: (a) a first layer of kraft paper; (b) a layer of controlled gas permeable polymer with no natural pinholes therein; and (c) a second layer of kraft paper, said first and second layers of kraft paper sandwiching the layer of polymer between them.
 2. A paperboard combination as claimed in claim 1 wherein the gas permeability of the polymer is between about 50 and 100,000 cc³/m² 24 hr. 1 atm.
 3. A paperboard combination as claimed in claim 1 wherein the first layer of kraft paper, the layer of polymer and the second layer of kraft paper are affixed together in one step.
 4. A paperboard combination as claimed in claim 1 wherein the layer of polymer is extruded on one side of the first layer of kraft paper and the second layer of kraft paper is then affixed to the layer of polymer on the side opposite the first layer of kraft paper.
 5. A paperboard combination as claimed in claim 1 wherein the layer of polymer is preformed and is then laminated between the first layer of kraft paper and the second layer of kraft paper.
 6. A paperboard combination as claimed in claim 1 wherein the layer of polymer is low density polyethylene.
 7. A paperboard combination as claimed in claim 1 wherein the layer of polymer is high density polyethylene.
 8. A paperboard combination as claimed in claim 1 wherein the layer of polymer is a copolymer of low density polyethylene and ethylenebutylacetate.
 9. A paperboard combination as claimed in claim 1 wherein the overall permeability of the paperboard combination is regulated by regulating the thickness of the polymer and the processing conditions in forming the paperboard combination so that no natural pinholes are formed.
 10. A paperboard combination as claimed in claim 1 wherein the overall permeability of the paperboard combination is regulated in part by regulating the the composition of the polymer layer and the type of kraft paper.
 11. A paperboard combination as claimed in claim 1 wherein the polymer is selected from the group consisting of ethylene vinylacetate (EVA), ethylbutyl acetate (EBA), a crosslinked ionomer resin, cast polyester (PET), a polyamide and polycarbonate (PC).
 12. A paperboard combination as claimed in claim 1 wherein a portion of the first layer of kraft paper and a corresponding opposite portion of the second layer of kraft paper are absent so that the intermediate polymer layer forms a window.
 13. A paperboard combination as claimed in claim 12 wherein the polymer layer at the location of the window includes an anti-fog agent.
 14. A paperboard combination as claimed in claim 13 wherein the anti-fog agent is silicone, non-ionic surfactant or oil-based material.
 15. A paperboard modified atmosphere package container suitable for packaging fruits and vegetables under refrigerated modified atmosphere conditions comprising: (a) a container constructed of an erected paperboard blank having flaps, side panels, end panels, base panel and lid panel with flaps, side panels and end panels formed therein and fold and joint lines impressed therein, said paperboard blank being constructed of: (i) a first layer of kraft paper; (ii) a layer of polymer having a gas permeability which permits gas to be transmitted in either direction through the polymer at prescribed levels; and (iii) a second layer of kraft paper, said first and second layers of kraft paper sandwiching the polymer between them; and (b) glue applied to intersecting joints, flaps, side panels, end panels, and lid panels to provide a hermetic seal to the erected container, said container having a required overall permeability which permits gas to be transmitted into or out of the container.
 16. A container as claimed in claim 15 wherein the flaps, side panels, end panels and base panel of the container are formed from one piece of paperboard and the lid panel with flaps, side panels and end panels are formed from a second piece of paperboard.
 17. A container as claimed in claim 15 wherein the glue is hot melt glue which is applied to cover exposed and intersecting edges of the joints, flaps, side panels, end panels and lid panels.
 18. A container as claimed in claim 15 wherein the glue is a foamed glue which is applied to cover exposed and intersecting edges of the joints, flaps, side panels, end panels and lid panels.
 19. A container as claimed in claim 15 wherein the glue is cold set water resistant glue which is applied to cover exposed and intersecting edges of the joints, flaps, side panels, end panels and lid panels.
 20. A container as claimed in claim 15 wherein at least some edges of the joints, flaps and panels are sealed with tape.
 21. A container as claimed in claim 15 wherein at least some of the joints, flaps and panels are sealed with glue and tape.
 22. A container as claimed in claim 15 wherein a first side of the first layer of kraft paper, opposite the side adjacent the polymer layer, is coated with a polymeric coating.
 23. A container as claimed in claim 15 wherein edges of flaps intersecting with sides of the container are sealed with glue, and the flaps are bent around the corners of the glued edges, and are glued at a second location.
 24. A container as claimed in claim 15 wherein a portion of the first layer of kraft paper and an adjacent portion of the second layer of kraft paper are absent to thereby form a window in the container.
 25. A container as claimed in claim 15 wherein the layer of polymer is at least partially permeable to oxygen and carbon dioxide.
 26. A container as claimed in claim 15 wherein exposed edges of flaps and sides are folded, and the exposed edges of the paperboard are sealed with glue.
 27. A container as claimed in claim 15 wherein the lid panel has a window.
 28. A container as claimed in claim 27 wherein the window has an anti-fog agent on its surface.
 29. A container as claimed in claim 27 wherein the anti-fog agent is silicone, non-ionic surfactant or oil-based material.
 30. A container as claimed in claim 15 wherein the layer of polymer is selected so that the container has a gas permeability which is within a predetermined gas transmission range which is selected in accordance with the respiration rate and characteristics of produce to be packaged in the container.
 31. A container as claimed in claim 15 wherein the layer of polymer is flexible and has selected gas and moisture permeabilities.
 32. A paperboard as claimed in claim 15 wherein the gas permeability of the polymer is between about 50 and 100,000 cc³/m² 24 hr. 1 atm.
 33. A container as claimed in claim 15 wherein the container is filled with produce, a modified atmosphere is injected into the container, and the container is held at a temperature between about 1° C. to 12° C.
 34. A paperboard modified atmosphere package container combination suitable for packaging fruits, vegetables and cut flowers under refrigerated modified atmosphere conditions comprising: (a) a container constructed of erected paperboard lid and body blanks having joints, flaps, side panels, end panels, base panels and lid panels formed therein and fold and joint lines impressed therein, said paperboard blanks being constructed of: (i) a first layer of kraft paper; (ii) a layer of polymer having a gas permeability which permits gas to be transmitted in either direction through the polymer at prescribed levels; and (iii) a second layer of kraft paper; glue applied to intersecting joints, flaps, side panels, end panels, base panels and lid panels to provide a hermetic seal to the erected container; and (b) a Euro tray with recessed sides and ends holding the container.
 35. A container combination as claimed in claim 34 wherein a plurality of erected containers containing fruit or vegetables are positioned in the Euro tray.
 36. A container combination as claimed in claim 34 wherein the Euro tray has recessed ends and sides to facilitate circulation of cool air when the Euro trays are stacked upon one another.
 37. A paperboard combination as claimed in claim 34 wherein a portion of the first layer of kraft paper and a corresponding opposite portion of the second layer of kraft paper are absent so that the intermediate polymer layer forms a window.
 38. A paperboard combination as claimed in claim 34 wherein the polymer layer at the location of the window includes an anti-fog agent.
 39. A paperboard combination as claimed in claim 34 wherein the anti-fog agent is silicone, non-ionic surfactant or oil-based material. 